Enforce consistent prefix for numbered lists

Enforced another Markdown rule: https://github.com/markdownlint/markdownlint/blob/master/docs/RULES.md#md029---ordered-list-item-prefix, which aligns with our Styleguide for numbered lists: https://docs.gitlab.com/ee/development/documentation/styleguide.html#list-items.

Numbered lists should always be prefixed with 1..

Relates to: https://gitlab.com/gitlab-com/gitlab-docs/issues/287. Related to https://gitlab.com/charts/gitlab/issues/1529

CC @gl-docsteam